The present invention generally relates to a method for making reticles for use in a photolithographic process and reticles formed and more particularly, relates to a method for making reticles that have reduced particle contamination problem by providing a recessed slot in a top surface of a quartz plate for holding an adhesive and reticles formed by such method.
A reticle, by definition, is a transparent ceramic substrate that is coated with a metallic layer to form a pattern for an electronic circuit. The reticle is generally used in an imaging step during a photolithographic process where a pattern of a circuit is reproduced on the surface of an electronic substrate, i.e. on a wafer surface. In a general sense, a reticle, or a photomask, is a template for wafer lithography, much like a photographic negative. The fabrication process for a photomask is similar to that of wafers, i.e., a layer of a suitable metal such as chrome is deposited on a plate of pure quartz, called a photoblank. A layer of a photoresist and anti-reflective coating are also deposited. The combined layer is then exposed by a photomask production tool, commonly known as an e-beam or a laser-based pattern generation tool. These tools directly write onto the photomask the original stored IC design that was inputted by the circuit designer. The photoresist layer is then developed and the chrome layer is etched in the specific pattern. Where the chrome has been etched, pure quartz is left through which visible or UV light can penetrate. The remaining photoresist layer is then stripped, while the mask is inspected for integrity and ready for use.
The fabrication process for the reticle, or photomask, starts with the selection of a photoblank. Quartz is the most commonly used photoblank material due to its excellent optical and process characteristics. The mask size can range from a 3xe2x80x3 square with a thickness of the mask ranging between 6 mm to 25 mm. As the area of the glass is increased, the thickness of the glass must also be increased to support the weight of the glass.
A pellicle, or an ultra-thin transparent film, is placed on top of a pellicle frame over the photomask to seal or protect the photomask from contamination and damage. The pellicle protects the photomask in such a way that particles fallen onto and adhering to the pellicle surface are kept at a distance far enough away from the wafer surface such that they are not imaged onto the wafer surface during the pattern exposure process. The pellicle membrane film is coated with an anti-reflective coating to improve the optical performance at deep UV wavelengths. The pellicle membrane film can be formed of a polymeric material such as Mylar(copyright) which must be stable enough to retain its shape both over a long period of time and prolonged exposure to UV light.
A conventional reticle with a pellicle protection film covered on top is shown in FIG. 1. The reticle 10 is constructed by a base plate 12 formed of quartz, a metallic layer 14 formed of chrome on top of the quartz plate 12. A pellicle frame 16 is positioned on top of the base plate 12 and adhered to a top surface 18 by an adhesive bead layer 20. A pellicle membrane film 22 is placed on top of the pellicle frame 16 to seal and shield the photomask, i.e. the patterned chrome layer 14, from contamination and damage.
In connection with the scanner tool is an integrated reticle inspection system (IRIS) which is used to in-line scan both surfaces of the quartz side and the pellicle side prior to the processing of a wafer lot. When the IRIS inspection system detects particles on the surface of the pellicle or the quartz plate, a dry nitrogen gas is blown onto the pellicle and the quartz plate to remove the particles. However, during the nitrogen blowing process, not only particles are blown away, some contaminating particles generated by an aged polymeric adhesive bead 20 may also be blown into the cavity 24 under the pellicle which become a contamination source for the mask layer 14. The contamination problem is especially severe when the polymeric based adhesive 20 is aged after prolonged usage and numerous UV exposure processes.
It is therefore an object of the present invention to provide a reticle for holding a mask thereon without the drawbacks or shortcomings of the conventional reticles.
It is another object of the present invention to provide a reticle for holding a mask thereon that has a reduced particle contamination problem.
It is a further object of the present invention to provide a reticle for holding a mask thereon wherein the mask is protected by a pellicle without a particle contamination problem.
It is another further object of the present invention to provide a reticle for holding a mask thereon without a particle contamination problem generated by an adhesive bead that bonds a pellicle frame to the reticle.
It is still another object of the present invention to provide a reticle that has a recessed slot formed in a top surface of the quartz plate for holding an adhesive therein and for bonding to a pellicle frame.
In accordance with the present invention, a method for making a reticle that has reduced particle contamination problem and a reticle thus made are disclosed.
In a preferred embodiment, a reticle for holding a mask thereon with reduced particle contamination problem can be provided which includes a base plate formed of an optically transparent material capable of surviving a service temperature of at least 50xc2x0 C., the base plate has a recessed slot in a top surface enclosing an area that is at least an area of a mask formed on the base plate; an adhesive partially filling the recessed slot in the top surface of the base plate such that a top surface of the adhesive is at least 0.5 mm below the top surface of the base plate; a pellicle frame mounted in the recessed slot with a bottom end of the pellicle frame encapsulated in the adhesive and a top end extended vertically away from the top surface of the base plate; and a film covering the top end of the pellicle frame forming a hermetically sealed chamber cavity between the pellicle frame, the top surface of the base plate and the transparent film for keeping the mask formed on the top surface of the base plate in a substantially particle-free environment.
In the reticle for storing a mask with reduced particle contamination, the base plate may be formed of quartz. The base plate may have a thickness of at least 6 mm, while the recessed slot may have a depth of at least 2 mm. The mask may be formed of a metal that includes chromium, the adhesive may be formed of a polymer-based adhesive. The pellicle frame may be formed of a metal such as stainless steel. The film that covers the top end of the pellicle frame may be a polymeric film. The top surface of the adhesive may be at least 1 mm below the top surface of the base plate.
The present invention is further directed to a method for making reticles with reduced particle contamination which can be carried out by the operating steps of first providing a base plate that is formed of an optically transparent material that has a mask formed on a top surface; forming a recessed slot in the top surface of the base plate enclosing an area that is at least an area occupied by the mask; positioning a bottom edge of a pellicle frame in the recessed slot; filling the recessed slot with an adhesive encasing the bottom edge of the pellicle frame such that a top surface of the adhesive is at least 0.5 mm below the top surface of the base plate; and sealing a top edge of the pellicle frame with an optically transparent film forming a hermetically sealed chamber cavity between the pellicle frame, the top surface of the base plate and the optically transparent film such that the mask is kept in a substantially particle-free environment.
The method for making reticles with reduced particle contamination problem may further include the step of forming the base plate in quartz, the step of forming the mask on the top surface of the base plate in a metal that includes chromium, and the step of forming the recessed slot to a depth of at least 2 mm. The method for may further include the step of forming the base plate to a thickness of at least 6 mm, and preferably to a thickness of at least 10 mm. The method may further include the step of filling the recessed slot with a polymeric-based adhesive, or with an adhesive until a top surface of the adhesive is at least 1 mm below the top surface of the base plate. The method may further include the step of forming the pellicle frame in a rigid, non-particle-generating material, or the step of forming the pellicle frame in a stainless steel. The method may further include the step of forming the optically transparent film in a polymeric material.